In general power circuits having a function of detecting an abnormal condition, an overshoot of the output voltage occurs after recovery from the abnormal condition. The overshoot of the output voltage occurs due to a response delay of the power circuit to load change or due to the efficiency limit of an error amplifier circuit used for amplifying a voltage difference between a voltage based on the output voltage and a reference voltage. However, the overshoot of the output voltage is particularly likely to occur along with an expansion of the difference between the input and output voltages and a decrease in the capacity of an output capacitor.
It is a common structure for power circuits having an overcurrent protection function as a function of detecting an abnormal condition to have an error amplifier circuit (or a comparison circuit) for detecting an overshoot of the output voltage in addition to an error amplifier circuit (or a comparison circuit) for stabilizing the output voltage (see Patent Document 1, for example).
However, it is difficult to create a minute voltage difference between a threshold of the error amplifier circuit (or the comparison circuit) for stabilizing the output voltage and a threshold of the error amplifier circuit (or the comparison circuit) for detecting an overshoot of the output voltage.
Accordingly, some conventional technologies have adopted a structure of reducing an overshoot of the output voltage by cutting off the electrical connection between an error amplifier and a phase compensation capacitor at the time of overcurrent detection and discharging an electric charge of the phase compensation capacitor (see Patent Document 2, for example), instead of reducing an overshoot of the output voltage by reducing a difference between the threshold of the circuit for stabilizing the output voltage and the threshold of the circuit for detecting an overshoot of the output voltage. With this structure, it is not required to improve the capacity of the error circuit the comparison circuit) used for stabilizing the output voltage more than necessary.
Patent Document 1: Japanese Laid-open Patent Application Publication No. H9-117131 (paragraphs [0101] to [0102]; FIGS 1 and 2, for example)
Patent Document 2: Japanese Laid-open Patent Application Publication No. 2009-253992 (FIGS. 1, 3, 4 and 7, for example)
With the structure as disclosed by Patent Document 2, it is required neither to provide the error amplifier circuit (or the comparison circuit) for detecting an overshoot of the output voltage nor to improve the capacity of the error amplifier circuit (or the comparison circuit) for stabilizing the output voltage more than necessary. However, in order to determine the timing for reconnecting the phase compensation capacitor, which has been electrically cut off from the error amplifier circuit, the structure still requires an output-voltage recovery detection circuit for detecting that the output voltage has recovered to a set voltage from being smaller than the set voltage.
In addition, even if the overcurrent detection takes a small amount of time, a large undershoot of the output voltage occurs at the time of the reconnection of the phase compensation capacitor to the error amplifier circuit due to a discharge of an electric charge of the phase compensation capacitor. Subsequently, an overcurrent is detected again at the time of recovery from the undershoot of the output voltage, which in turn results in instability of the output voltage.